Thermoplastic materials that possess a wide range of improved properties suitable for many diversified applications are obtained by polymerization of vinyl aromatic monomers in the presence of elastomeric materials. Commercial extrusion grade impact polystyrene toughened with polybutadiene rubber is used for an increasing number of applications requiring a tough, high quality, easily extruded, easily formed, and cost-competitive material.
Commercially important impact resistant polymers can be produced by polymerizing a major amount of vinyl aromatic compounds with a minor amount of rubber. Numerous different types of vinyl aromatic compounds and rubbers may be used, and are well known to those skilled in the art. In a polymerizing mixture, some of the vinyl aromatic compound polymerizes to form homopolymer, while the rubber may react with either such homopolymer or with monomer to form grafted copolymer. Impact resistant polymers appear to comprise a mixture of homopolymer and copolymer wherein the copolymer is distributed throughout the mass. Only a small amount of rubber is, generally, used. The amount of rubber used, typically, is about 10 percent by weight or less of the total polymers mass, but this is sufficient to impart impact strength to the total polymer mass.
With growth and sophistication of the impact resistant polymer industry has come increasing need for lower defects in most extrusion applications, particularly in thin film or coextrusion applications which are sensitive to appearance defects and defect-induced tears generated during extrusion and forming. It is well known that one major source of finished product defects is rubber gels or insoluble particles, such as crosslinked polybutadiene. While conditions during polymerization are closely controlled to minimize new gel formation, gels can be inherently present in the rubber and can harden during the polymerization process.
U.S. Pat. No. 4,230,835 to Richard C. Well describes a method of separating polybutadiene gels from styrene solutions by passing the solution through filter media of viscose rayon mat or felt on which gels collect. This method is reported to use viscose rayon mat or felt having a porosity of from about 15 to 50 micrometers. In testing, however, the patent states that it was decided to use a viscose felt with an opening size of less that 40 micrometers because about 40 micrometers is the smallest size particle visible to the naked eye.
It is an object of this invention to provide apparatus and processes which facilitate reduction of elastomer globules to small particles distributed evenly in liquid monomer and thus improving the quality of vinyl aromatic polymers produced therefrom by a mass thermal process.